The invention is directed to a method for controlling track following in a tape recorder. When recording in the oblique tracks of tapes used with analogue and digital tape recording devices, either video or audio, it is necessary for each of the two heads to exactly find its track during the reproduction of the video or audio. To this end, for VHS, a pulse is recorded on a longitudinal track of the magnetic tape when a particular head moves into alignment with the tape. During reproduction, the pulse is evaluated and serves to control the tape/head servo regulation. Entailed by the influence of tape stretching and adjustment tolerances, a means for adjusting the pulse position by the user, so-called tracking, is provided in such types of device.
The 8 mm video system contains a so-called ATF (automatic track following) circuit with which the track maintaining information is inserted into the channel in use. During recording, a predetermined sequence of specific low frequency signals, typically one per track, is interleaved in frequency gaps existing in the useful signal spectrum. During reproduction, the head reads user information from the track, and also, as a result of the low azimuthal coupling of the low frequency signals, track maintaining information is free from the two adjacent tracks. The track following is controlled by amplitude comparison of the signals from right and left neighboring tracks.
In the DAT system (Digital Audio Tape) the digital audio signals, the useful signal, and the track following signal are recorded at separate locations, whereby a so-called "run-in" region which is used for the synchronization of the clock rate recovery, is placed in front of the useful signal and the ATF frequencies are obtained during recording by division of the signal clock rate. For the digital recording of video, especially for HDTV, the recording data rate is high and spatially separate recording of signals on the tape is hardly tenable since the amount of tape surface which is needed is then intolerably high.
The special generation of the ATF signal is described in IEEE Transactions on Consumer Electronics, August 88, page 597-605. In this technique a "run-in" region is associated with the clock rate recovery and the ATF information is formed by a cyclical variation of the DC component of the data items in the channel. The variation of a digital sum in each cycle of 10 bits permits the generation of triangular signals having various basic frequencies, each of different amplitudes, as permanent additional functions to the 8/10 channel modulation (matching the input signal to the capabilities of the recording channel).
The various know solutions have, to a greater or lesser extent, several disadvantages. In general, only triangular shaped signals can be produced which result in a large content of higher harmonics and a correspondingly high harmonic distortion factor. Generally, different frequencies have undesirably different amplitudes even during the generation. If a permanent ATF signal is also produced, balancing of priorities is necessary if two contradictory requirements are to be fulfilled. Errors in the ATF sequence can arise because the error rate of the useful signal should not rise e.g. through over stepping of the run length and the loss of synchronization associated therewith. The possibilities for channel matching are in general distinctly limited. Without the inclusion of ATF, only 256 (8 bits) are selected from 1024 possible output combinations (10 bits) in the 8/10 modulation.
These 256 bits are selected with regard to their disparity to 0 (sum of all H bits--sum of all L bits), which is not always possible, and are optimized as regards their run length so that the low and high frequency components are reduced. The unused combinations (code words) 1024-256 can be ignored. Their appearance during reproduction is an indication for the occurrence of an error. This so-called "erasure" may be used to advantage for error correction. By inclusion of the ATF, the code word range is appreciably larger, and especially advantageous code words (disparity=0) can be used infrequently. The additional amount of circuitry is generally substantial since the digital sum value is controlled by two variable sources. The basic 8/10 conversion would give an optimized data word which would still have to be tested as to its suitability as an ATF code word (checking of the disparity and, if necessary, a search for a new code word).
The object of the invention is to develop a method of controlling track following in a recorder without the need for recording various ATF frequencies and which makes the production of an ATF signal having a smaller number of harmonics and a low harmonic distortion factor possible.